1. Field of the Invention
This invention relates to a non-aqueous electrolyte secondary cell. More particularly, it relates to a non-aqueous electrolyte secondary cell having a cleavage valve which, in case of an increased internal pressure, releases the pressure by exploiting this increased internal pressure.
2. Description of the Related Art
Recently, with coming into widespread use of portable equipments, such as video cameras or cassette recorders, there is raised an increasing demand for secondary cells that can be repeatedly used in place of the disposable primary cells.
The majority of currently used secondary cells are nickel-cadmium cells employing an alkali electrolyte liquid. However, this aqueous solution type cell has a discharging potential as low as approximately 1.2 V and a large cell weight and volume such that it cannot satisfactorily meet the demand for a cell having a high energy density. The aqueous solution type cell also has a drawback that it has a self discharge rate as high as approximately 20% per month at ambient temperature.
Thus, a non-aqueous electrolyte secondary cell employing a non-aqueous solvent as the electrolyte solution and also employing light metals, such as lithium as the negative electrode, is under investigations. This non-aqueous electrolyte secondary cell has a voltage as high as 3 V or higher, a high energy density and a low self-discharge rate.
However, this sort of the secondary cell also can hardly be put to practical use because of a drawback that metal lithium used for the negative electrode undergoes dendritic crystal growth to contact with the positive electrode, due to repetition of charging/discharging, as a result of which shorting tends to be produced in the cell to lower the service life of the cell.
In order to overcome this drawback, a non-aqueous electrolyte secondary cell employing, as a negative electrode, an alloy obtained on alloying lithium with other metals, is also under investigations.
In this case, however, the alloy is turned into comminuted particles, due to repetition of charging/discharging, thus again lowering the service life of the cell.
Under this situation, there is proposed a non-aqueous electrolyte secondary cell employing a carbonaceous material, such as coke, as an active material for the negative electrode.
This secondary cell is free of the above-mentioned drawbacks in the negative electrode and hence is superior in cyclic service life characteristics. If a lithium transition metal complex oxide is used as an active material for the positive electrode, the service life of the cell is prolonged to enable realization of the non-aqueous electrolyte secondary cell having the desired high energy density.
Meanwhile, in a non-aqueous electrolyte secondary cell employing the carbonaceous material as a negative electrode, there is required a pressure relieving mechanism for promptly releasing the pressure when the cell is subjected to an abnormal temperature rise or combustion.
If this pressure relieving mechanism comes into operation to release the gas when the internal pressure in the cell reaches a predetermined pressure, there can be provided an extremely safe cell free from explosions or the like.
It is therefore an object of the present invention to provide an extremely safe non-aqueous electrolyte secondary cell capable of promptly releasing an increasing internal pressure and which is free from explosion or the like.
As a result of repeated investigations towards achieving the above object, the present inventors have arrived at the information that a non-aqueous electrolyte secondary cell free from explosions or the like can be provided by providing a cleavage valve that is cleft in case of rise in the internal pressure of the cell and by optimizing the diameter of the opening of the valve in association with the internal capacity of the cell to optimize the operating pressure to carry out satisfactory gas release at the time of abnormal pressure increase or on combustion.
The present invention, completed on the basis of this information, resides in a non-aqueous electrolyte secondary cell having a cleavage valve which is realized by bonding a metal foil to an opening in the valve. The metal foil is cleft due to rise in the internal pressure in the cell to release the pressure. A value K corresponding to the internal cell volume in cm3 divided by the area of the opening in cm2 is such that 40xe2x89xa6Kxe2x89xa6100 xcexcm.
By selecting the K value to be a suitable value, the cleavage valve is in operation promptly when the internal pressure reaches a predetermined pressure in order to release the pressure. Thus, there is no risk of dilation or explosion of the cell to assure high operational safety.
Thus, the present invention provides an extremely safe cell free from explosions since gas release may be promptly realized on the occasion of rise in the internal pressure in the cell.